1. Field of the Invention
The present invention relates to a stepper motor, and more particularly, to a lens driving device for cameras and a stepper motor suitable for use in such a lens driving device.
2. Description of the Related Art
Motors having a structure which allows a motor to be easily disposed within the lens barrel of a camera as a lens driving device thereof have been proposed in, for example, U.S. Pat. Nos. 4,806,813 and 4,958,099.
Stepper motors for driving the shutter blade or photographic lens of a camera have been disclosed in, for example, Japanese Utility Model Laid-Open No. sho 63-113127. This stepper motor includes a rotor 92 which is divided in the circumferential direction thereof and magnetized radially, a plurality of stators 93 and 94, each of which has a plurality of magnetic poles disposed around the rotor 92 in such a manner that they face the rotor 92, and coils 95 and 96 disposed on parts of the stators 93 and 94 in order to magnetize the stators, respectively, as shown in FIG. 40. The entire stepper motor is disposed on a barrel base plate 91 substantially in the form of an arc so that it can be readily disposed within the lens barrel of the camera.
More specifically, the rotor 92 is mounted on the barrel base plate 91 in such a manner as to be rotatable. As shown in FIG. 40, the rotor 92 is divided into four portions in the circumferential direction. These four portions alternately have south and north magnetic poles. The stators 93 and 94 have magnetic poles 93a, 93b and 94a, 94b, respectively, which are disposed around the rotor in such a manner that they oppose the rotor. In the stepper motor, the rotor is rotated in either direction by switching over energization of the coils. The rotation of the rotor is used to drive the shutter blade (not shown) or photographic lens (not shown).
However, in the above-described conventional stepper motor, the respective magnetic poles 93a, 93b, 94a and 94b of the stators 93 and 94 must be disposed two-dimensionally, as shown in FIG. 40. Thus, the reduction in the width I of the stepper motor is limited, in turn limiting the reduction of a diameter D of the lens barrel.
Also, the above-described conventional stepper motor has the following drawback. When the photographic lens is driven by the stepper motor, movement of the lens must be stopped during the exposure operation. Generally, the rotor is stopped at predetermined rotational positions by the cogging torque generated between the magnetized portions and the stators. However, there are only four rotational positions where the rotor can be retained stably by the cogging torque. These four rotational positions, shown in FIGS. 40 and 41, are those obtained each time the rotor is rotated by 90.degree. (FIG. 40 shows two states out of phase by 180.degree. among these four rotational positions, and FIG. 41 shows the other two positions out of phase by 180.degree.). In this stepper motor, the rotor can also be retained at four positions, shown in FIGS. 42 and 43, obtained each time the rotor is rotated by 90.degree. by energizing the coils 95 and 96. However, since the cogging torque obtained at each of the rotational positions shown in FIGS. 42 and 43 is very small, when the coils are deenergized, the rotor may not be retained stably. Hence, when the positions shown in FIGS. 42 and 43 are used as those at which the movement of the photographic lens is stopped, the coils must be kept energized so that the rotor can be retained at the positions shown in FIGS. 42 and 43 by the electromagnetic force generated by the energization of the coils.
To perform exposure control during operation of the camera, an actuator for driving an exposure control mechanism (not shown) must be energized. Taking into account the fact that an electric cell is used as the power source of the camera, it is not desirable to energize the actuator for moving the photographic lens, i.e., the coils 95 and 96, during the exposure operation.
In the lens driving device for cameras, it is desirable to move the lens along the optical axis accurately by sliding a guide shaft for retaining a lens frame along a guide hole disposed on the optical axis. However, in conventional lens driving devices disclosed in, for example, Japanese Patent Laid-Open No. hei 3-180822 and Japanese Utility Model Laid-Open No. hei 4-50810, a guide portion for positioning the lens and a threaded portion for driving the lens are disposed separately.
In the above-described lens driving devices, since the force which drives the lens or lens frame along the optical axis acts on a site different from the guide portion, the lens or lens frame may be inclined, or the lens frame may be deflected. These increase the friction of the guide portion and increase the driving load.